This invention relates to an ink jet printing method which employs a porous receiver and an ink jet composition which provides improved light and dark stability.
Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. The inks used in ink jet printers are generally classified as either dye-based or pigment-based.
A dye is a colorant which is molecularly dispersed or solvated by a carrier. The carrier can be a liquid or a solid at room temperature. A commonly used carrier is water or a mixture of water and organic co-solvents. Each individual dye molecule is surrounded by molecules of the carrier. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor light-fastness. When water is used as the carrier, such inks also generally suffer from poor water fastness.
U.S. Pat. Nos. 4,246,154 and 5,852,074 relate to an ink jet ink composition comprising a water-insoluble dye dispersed in a water-dispersible polymer. However, there is a problem with this ink in that when it is printed onto a conventional receiver, the dry time is slow.
U.S. Pat. No. 4,460,637 relates to a porous ink jet receiver element. However, there is a problem with this element in that when it is printed with a conventional aqueous dye-based ink, the printed image has poor light and dark stability.
It is an object of this invention to provide an ink jet printing method which provides an image which has a fast dry time. It is another object of this invention to provide an ink jet printing method which provides an image which has improved light and dark stability.
These and other objects are achieved in accordance with this invention which relates to an ink jet printing method, comprising the steps of:
A) providing an ink jet printer that is responsive to digital data signals,
B) loading the printer with ink-receptive elements comprising a support having thereon a continuous, coextensive porous ink-receptive layer,
C) loading the printer with an ink jet ink composition comprising an aqueous-dispersible polyester having contained therein a water-insoluble dye, and
D) printing on the ink-receptive element using the ink jet ink in response to the digital data signals.
The support for the ink-receptive element can be paper or resin-coated paper, or plastics such as a polyester-type resin such as poly(ethylene. terephthalate), polycarbonate resins, polysulfone resins, methacrylic resins, cellophane, acetate plastics, cellulose diacetate, cellulose triacetate, vinyl chloride resins, poly(ethylene naphthalate), polyester diacetate, various glass materials, etc. The thickness of the support employed in the invention can be, for example, from about 12 to about 500 xcexcm, preferably from about 75 to about 300 xcexcm.
In a preferred embodiment of the invention the continuous, coextensive porous ink-receptive layer contains inorganic particles such as silica, alumina, titanium dioxide, clay, calcium carbonate, barium sulfate, or zinc oxide. In another preferred embodiment, the porous ink-receptive layer comprises from about 20% to about 90% inorganic particles and from about 10% to about 80% polymeric binder, such as gelatin, poly(vinyl alcohol), poly(vinyl pyrrolidinone) or poly(vinyl acetate). The porous ink-receptive layer can also contain polymer micro-porous structures without inorganic filler particles as shown in U.S. Pat. Nos. 5,374,475 and 4,954,395.
A broad range of water-insoluble dyes may be used in the invention such as an oil dye, a disperse dye, a solvent dye, as disclosed in U.S. Pat. Nos. 4,246,154 and 5,852,074, or a metal-complex dye, such as the water-insoluble analogues of those described in U.S. Pat. Nos. 5,997,622 and 6,001,161, i.e., a transition metal complex of an 8-heterocyclylazo-5-hydroxyquinoline.
The dye-containing aqueous-dispersible polyester used in the invention can be prepared by dissolving the dye in a water-miscible organic solvent, mixing the solution with the aqueous-dispersible polyester and then removing the solvent. Useful water-miscible organic solvents are water-miscible alcohols, ketones and amides, tetrahydrofuran, N-methyl-2-pyrrolidone, dimethylsulfoxide and mixtures thereof, such as acetone, ethyl alcohol, methyl alcohol, isopropyl alcohol, dimethylformamide, methyl-ethyl ketone etc.
The ink jet ink containing the water-dispersible polyester employed in the invention consists of water as a continuous phase and dye-containing polyester as a dispersed phase. In a preferred embodiment of the invention, the aqueous-dispersible polyester meets the following test: At 25xc2x0 C., the aqueous-dispersible polyester must: (a) be capable of forming a stable dispersion with water at a concentration of from 0.2 to 50 percent by weight, preferably 1 to 20 percent by weight, and (b) when 100 ml of the aqueous-dispersible polyester is then mixed in an equal volume of the water-miscible organic solvent described above, stirred and allowed to stand for 10 minutes exhibit no observable coagulation of the aqueous-dispersible polyester. In order to be useful in the ink, the aqueous-dispersible polyester should have an average particle size of  less than 1 xcexcm, preferably  less than 0.2 xcexcm.
In a preferred embodiment of the invention, the water-dispersible polyester contains dicarboxylic acid recurring units and diol recurring units consistent with the following general formula: 
wherein:
R1 and R2 each independently represents a saturated or unsaturated divalent hydrocarbon, or aromatic or aliphatic group or contains both aromatic and aliphatic groups, such as 1,2-phenylene, 1,3-phenylene, 1,4-phenylene, 1,4-naphthylene, 2,6-naphthylene, 4,4xe2x80x2-oxydiphenylene, 1,4-cyclohexylene, 1,2-ethylene, 1,4-butylene, and the like;
Z represents an ionic moiety derived from a sulfonic acid.
x represents a mole fraction from about 0.05 to about 0.8;
R3 represents S; an alkylene group of 1 to about 16 carbon atoms, a cycloalkylene group of 5 to about 20 carbon atoms; a cyclobisalkylene group of about 8 to about 20 carbon atoms; a bi- or tri-cycloalkylene group of about 7 to about 16 carbon atoms; a bi- or tri-cyclobisalkylene group of about 9 to about 18 carbon atoms; an arylenebisalkylene group of from 8 to about 20 carbon atoms or an arylene group of 6 to about 12 carbon atoms; or a carbinol-teiminated polydimethylsiloxane segment,
R4 and R5 each independently represents H, a substituted or unsubstituted alkyl group of 1 to about 6 carbon atoms or a substituted or unsubstituted aryl group of about 6 to about 12 carbon atoms; and
m and n each independently represents an integer from 0-4.
In a preferred embodiment of the invention, the dicarboxylic acid recurring units contain an ionic moiety derived from a sulfonic acid and includes the following: 
wherein:
M+represents alkali metals, such as Li, Na and K; ammonium groups such as ammonium, trimethylammonium, triethylammonium, tetraalkylammonium, aryltrialkylammonium, hydroxyalkylammonium, etc., phosphonium groups such as triphenylphosphonium or tetrabutylphosphonium; heteroaromatic ammonium groups such as pyridinium, imidazolium or N-methylammonium; sulfonium groups; guanidinium groups; amidinium groups, etc. Preferably, M+is an alkali metal, for example Na+.
In a preferred embodiment of the invention, the nonionic dicarboxylic acid recurring unit is isophthalic acid, and is present in a mole fraction from approximately 0.2 to approximately 0.95, more preferably from approximately 0.6 to approximately 0.90.
In another preferred embodiment of the invention, the ionic dicarboxylic acid recurring unit is 5-sulfoisophthalic acid, and is present in a mole fraction from approximately 0.05 to 0.8, more preferably from approximately 0.01 to 0.4.
Preferred diol recurring units useful in the invention include one or more selected from ethylene glycol, diethylene glycol, triethylene glycol, thiodiethanol, 1,4-cyclohexanedimethanol, bisphenol A, trans- 1,4-cyclohexanediol, dodecanediol, cis-exo-2,3-norbornanediol, 5-norbornene-2,2-dimethanol, hydroquinone bis(2-hydroxyethylether), carbinol terminated polydimethylsiloxane, MW=1000 (DMS-C15), (Gelest Inc.), etc. More preferred are diethylene glycol and 1,4-cyclobexanedimethanol.
In addition to the dicarboxylic acid recurring units mentioned above, other carboxylic acids or their functional equivalents can be incorporated in the polyester useful for the invention. These include monocarboxylic acids such as, for example, benzoic acid, substituted benzoic acids, naphthoic acid, substituted naphthoic acids, cyclohexanecarboxylic acid, hexanoic acid, lauric acid, sulfobenzoic acid salts, etc. Other carboxylic acids or their functional equivalents include maleic acid, fumaric acid, citraconic acid, itaconic acid, mesaconic acid, and other carboxylic acids containing ethylenic unsaturation.
Still other carboxylic acids or their functional equivalents are pyromellitic acid, trimellitic acid, trimesic acid, and other polycarboxylic acids.
In addition to the diol recurring units mentioned above, other hydroxy-containing compounds or their functional equivalents can be incorporated in the polyester useful for the invention. These include phenols, hydroxybenzoic acid, cyclohexanol, lauryl alcohol, alcohols or diols containing ethylenic unsaturation, and polyols such as glycerol, trimethylol propane, and pentaerythritol.
The aqueous dispersible polyester employed in the invention may be, for example, Eastman AQ(copyright) polyesters produced by the Eastman Chemical Company. The three polyesters, Eastman AQ 29, AQ 38, and AQ 55 are composed of varying amounts of isophthalic acid, sodium sulfoisophthalic acid, diethylene glycol, and 1,4-cyclohexanedimethanol. These thermoplastic, amorphous, ionic polyesters are prepared by a melt-phase condensation polymerization at high temperature and low pressure, and the molten product is extruded into small pellets. The solid polymer disperses readily in water at 70xc2x0 C. with minimal agitation to give translucent, low viscosity dispersions containing no added surfactants or solvents. Varying the amount of ionic monomers, i.e., sulfoisophthalic acid, can control the particle size. The particle sizes range from 20 to 100 nm.
A humectant is added to the composition employed in the process of the invention to help prevent the ink from drying out or crusting in the orifices of the ink jet printhead. Polyhydric alcohols useful in the composition of the invention for this purpose include, for example, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, tetraethylene glycol, polyethylene glycol, glycerol, 2-methyl-2,4-pentanediol, 1,2,6-hexanetriol and thioglycol. As noted above, the humectant is employed in a concentration of from about 10 to about 50% by weight. In a preferred embodiment, diethylene glycol or a mixture of glycerol and diethylene glycol is employed a concentration of between 10 and 20 wt. %.
A co-solvent can also be employed in the composition employed in the process of the invention. The selection of a co-solvent depends on the requirements of the specific application, such as desired surface tension and viscosity, the selected colorant, drying time of the ink jet ink, and the type of paper onto which the ink will be printed. Representative examples of water-soluble co-solvents that may be selected include (1) alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol, and tetrahydrofufuryl alcohol; (2) ketones or ketoalcohols such as acetone, methyl ethyl ketone and diacetone alcohol; (3) esters, such as ethyl acetate, ethyl lactate, ethylene carbonate and propylene carbonate; and (4) sulfur-containing compounds such as dimethyl sulfoxide and tetramethylene sulfone.
The ink has physical properties compatible with a wide range of ejecting conditions, i.e., driving voltages and pulse widths for thermal ink jet printing devices, driving frequencies of the piezo element for either a drop-on-demand device or a continuous device, and the shape and size of the nozzle.
A penetrant (0-10 wt. %) may also be added to the ink composition employed in the process of the invention to help the ink penetrate the receiving substrate, especially when the substrate is a highly sized paper. A preferred penetrant for the inks of the present invention is n-propanol at a final concentration of 1-6 wt. %.
A biocide (0.01-1.0 wt. %) may also be added to prevent unwanted microbial growth which may occur in the ink over time. A preferred biocide for the inks of the present invention is Proxel(copyright) GXL (Zeneca Colours Co.) at a concentration of 0.05-0.5 wt. %. Additional additives which may optionally be present in ink jet inks include thickeners, conductivity enhancing agents, anti-kogation agents, drying agents, and defoamers.
Commercially available ink jet printers use several different methods to control the deposition of the ink droplets. Such methods are generally of two types: continuous stream and drop-on-demand.
In drop-on-demand systems, a droplet of ink is ejected from an orifice directly to a position on the ink receptive layer by pressure created by, for example, a piezoelectric device, an acoustic device, or a thermal process controlled in accordance with digital data signals. An ink droplet is not generated and ejected through the orifices of the print head unless it is needed.
The following examples illustrate the utility of the present invention.